The present invention relates to an assembly for electrically protecting against a potential short-circuit or an overload in a power network having a system-determined, variable source-internal resistance. In particular, the present invention relates to an improvement in a short-circuit path, protected by way of an active or passive disconnecting element, in particular a fuse, in an on-board power network, which is supplied with energy by way of an electrochemical energy store. This disconnecting element is referred to below as a first protection device. It can be, for example, a fuse, pyro-fuse or the like.
On-board power networks of vehicles are frequently supplied with electrical energy by way of electrochemical energy stores or are buffered by means of the latter. In this context, the efficiency of the electrochemical energy stores fluctuates greatly with different operational variables and characteristic variables. For example, a state of aging of an electrochemical energy store is responsible for an increase in its internal resistance. In addition, the internal resistance is considerably dependant on temperature. The maximum power or voltage which can be output by the electrochemical energy store and the maximum current which can be output by the electrochemical energy store limit the output of energy here in the worse case so considerably that overcurrent or overvoltage protection measures sometimes respond too late or not at all. This is explained below with the example of a fuse. The electrical line cross sections through on-board power network sections which are protected by a fuse are usually dimensioned such that in the case of a predetermined current on the section the fuse responds before electric lines could be damaged. Owing to electrical insulation measures, the heating characteristics of the electric line sections and of the fuse differ greatly from one another. It is therefore possible that in the case of a first (relatively high) overcurrent, the fuse is supplied with electrical power sufficiently early, owing to which it responds and protects the electric line. However, if a current below the response current of the fuse is generated on a line section over a sufficiently long period of time, the electric line sections can be excessively loaded without the fuse being able to protect the electric line sections. As a result, the electric lines can be damaged and, in addition, give rise to consequential damage in the periphery. In the case of a short-circuit, the internal resistance of the electrochemical energy store which stores the electrical energy used for conducting the current can also limit the short-circuit current in such a way that the response current for the fuse is not reached. For example, an aged, heavily discharged and/or cold battery can limit the short-circuit current flowing in the case of a short circuit in such a way that over the long term a section of the electric line represents the weakest point in the on-board power network, instead of the fuse.
An object of the present invention is to eliminate the abovementioned disadvantages of the prior art.
According to the invention, an assembly for electrically protecting a first operating state against a predefined load, such as, for example, a short circuit in an on-board power network with an electrochemical energy store, is therefore proposed. The on-board power network is composed of line sections with different line cross sections and the supplied components.
The different line cross sections are referred to here as a first line section and a second line section, wherein the first line section has a relatively large cross section and the second line section has a smaller cross section in comparison therewith.
The abovementioned assembly also results in different operating states which are referred to here as first and second operating states. In the first operating state, the electrochemical energy store can have a higher internal resistance than in a second operating state. In other words, in the first operating state the electrochemical energy store can have a lower state of charge (SOC) and/or a higher rate of aging and/or a lower temperature than in the second operating state. In other words, in the first operating state a considerably reduced short-circuit current can be expected than in the second operating state, with the result that the line sections located in the circuit of the fuse are overloaded in the case of a short-circuit/overload, since the fuse does not trigger sufficiently quickly.
In this context, the triggering characteristic of a fuse whose strength limit is inadequately dimensioned for protecting a second line section of the potential short-circuit path for a first operating state of the energy store is, in other words, the case which is discussed above in conjunction with the prior art. It is provided that the fuse does not respond sufficiently quickly for all operating states of the energy store.
The “strength limit” of the fuse is to be understood within the scope of the present invention as meaning a maximum current or a maximum power at which the resistance of the fuse increases greatly or (virtually) infinitely. The second line section is a section within the potential short-circuit path which is not sufficiently protected by the fuse for all operating states of the energy store or of the on-board power network. Said line section can be, for example, a line section which has a smaller cross section than a first line section within the potential short-circuit path.
“Potential short-circuit path” is understood within the scope of the present invention to be that path via which the short-circuit current flows in the case of a short circuit. In other words, in the case of a short circuit, a portion of the on-board power network lines, together with the actual short circuit, are referred to together as a short-circuit path. According to the invention, a second protection device is therefore provided which is configured to protect the second line section against loading which occurs below the strength limit of the fuse in the first operating state. According to the invention, different measures are considered as the second protection device. In particular, control of contactors within the on-board power network can be provided to interrupt the short-circuit current. For this purpose, measurement of the current or power can be carried out for the short-circuit path, in particular for the second line section, by which measurement such an operating state is detected, which puts the second line section at risk but which cannot be averted by the fuse alone. In particular, the load can be detected as such which, on the one hand, actually places the second line section at risk and, on the other hand, is actually below the strength limit of the fuse. In other words, that operating state which indicates that the second line section is put at risk owing to a non-response of the fuse can be protected by the second protection device. For this purpose, the second protection device can include, for example, current sensors in the form of Hall sensors, inductive sensors or shunts. If a current which is in a predefined range is measured, the second protection device can open contactors and the short-circuit path can disconnect from the electrical energy store in this way.
In a further development, for example, the second protection device can include a cell undervoltage shutdown system for the electrochemical energy store. Cell undervoltage shutdown systems are known in the prior art for monitoring the electrical voltage of individual cells of an electrochemical energy store which is composed of a plurality of cells, and for averting the case of an undervoltage by shutting down the electrochemical energy store. An inventive use of a cell undervoltage shutdown system for protecting against a potential short-circuit path outside the electrical energy store is, however, not yet known. Of course, the cell undervoltage shutdown system can be modified according to the invention. For example, the variation of a cell voltage over time can be taken into account, with the result that an undervoltage case can be attributed to an undesired short-circuit with a high level of probability, and there is therefore a high level of probability that it is responsible for the cell undervoltage. The cell undervoltage shutdown system can detect here the short-circuit case insofar as the heavily overloaded short-circuit current gives rise to an increased voltage drop at the internal resistance of the electrochemical energy store or the respective cell. Alternatively or additionally, an overcurrent shutdown system, comprising a current measuring device, can be provided in the second protection device. The overcurrent shutdown system can (as stated above) determine a measured value for the short-circuit current, and open the contactors of the electrochemical energy store in response to detection of the first operating state. Since the mechanisms of the abovementioned second protection devices are present in contemporary topologies for storage protection, the invention can be implemented by way of simple, inventive adaptation of their actuation (for example, in the form of software code).
An overcurrent shutdown system is usually expected in the form of a cell undervoltage shutdown system in relatively high power ranges of the electrochemical energy store. In particular in the event of both an undervoltage shutdown system and an overcurrent shutdown system being provided according to the invention as the second protection device, it may also advantageously be provided that a lower current threshold value for the overcurrent shutdown system, and an upper voltage threshold value for the cell undervoltage shutdown system, are defined such that the response ranges of the overcurrent shutdown system and the cell undervoltage shutdown system at least adjoin one another. The triggering characteristic, chronological limiting values, current thresholds and voltage thresholds of the shutdown system owing to overcurrent shutdown system and undervoltage shutdown system are to be matched here to the triggering characteristic of the fuse and to the switching behavior of the disconnecting elements of the second protection device. Shutting down of the second protection device must therefore not take place too early in order to avoid overloading the contactors, and to permit triggering of the fuse. At the same time, the thermal overloading of the line in the case of a short circuit is to be avoided. In this way, reliable protection of the potential short-circuit path can be ensured over a wide working range of the electrochemical energy store. In order to improve the protection, in real application cases an overlap should be ensured between the operating states protected by the cell undervoltage shutdown system and those protected by the overcurrent shutdown system.
A possible and preferred dimensioning of the assembly mentioned above obeys the condition that the total number of cells of the energy store which are connected in series, when multiplied with a minimum cell individual voltage in the case of response of the cell undervoltage shutdown system divided by a lower current limiting value in the case of response of the overcurrent detection system, is higher than a maximum short-circuit resistance which can be assumed for the potential short-circuit path. In this context, the minimum cell individual voltage is understood to be such a voltage which, given an identical cell voltage of all the cells of the energy store, just brings about a response of the cell undervoltage shutdown system. This voltage represents the worst case which can occur in terms of the cell voltage during a short-circuit. Here, the maximum short-circuit resistance to be assumed is that resistance which occurs for the entire path conducting the short-circuit current, and is defined at the same time as a short-circuit case. In this way, particularly safe dimensioning is obtained by means of an overlap between the response ranges of the cell undervoltage shutdown system and those of the overcurrent shutdown system.
The second protection device is also preferably configured to have an inertia in the case of response such that, on the one hand, the potential short-circuit path is reliably protected even without response of the fuse and, on the other hand, outside the first operating state the fuse responds before the second protection device. Since, in fact, a response of the second protection device in the case of high current can lead to destruction or abnormal loading of the contactors, the high currents protection by response of the fuse is preferred. In order to provide inventive protection of a wide operating range, it is, however, necessary to make reliable use of the second protection device in conjunction with the contactors below a strength limit of the fuse.
It is also preferred here that the inertia of the second protection device is suitable for suppressing a response process in the case of brief current peaks, in particular even corresponding brief regulating interventions. In other words, in the response range of the second protection device, brief excessive increases of respectively defined threshold values are tolerated without the second protection device responding. Therefore, the second protection device can be dimensioned here as a function of electrical characteristic variables of the respective on-board power network which are to be evaluated as “normal”.
It is also preferred here that the inertia of the second protection device is suitable for differentiating an overload situation or short-circuit situation of the first and second line sections from a maximum operating situation of the first line section, and therefore for actually triggering only in the case of an overload and/or short circuit.
The second line section preferably has a smaller line cross section than a first line section of the same short-circuit path. The second line section is, as a result, given smaller electrical dimensions and forms, compared with the first line section, a section which is at a high probability of being put at greater risk in the case of a short-circuit current.
According to a further aspect of the present invention, a vehicle, comprising an electrochemical energy store and an assembly as discussed above, is proposed. The locomotion means can be, for example, an electric drive vehicle (PHEV, HEV, EV etc.). In this context, the electrochemical energy store can be configured as a traction battery and at least part of the potential short-circuit path can be configured as part of an on-board power network section which is configured to distribute the traction energy.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.